f DRY payment secret checks

Author: TheBlueMatt

lightning-invoice/src/lib.rs

@@ -1023,6 +1023,13 @@ impl Invoice {
 			return  Err(SemanticError::MultipleDescriptions);
 		}
 
+		self.check_payment_secret()?;
+
+		Ok(())
+	}
+
+	/// Checks that there is exactly one payment secret field
+	fn check_payment_secret(&self) -> Result<(), SemanticError> {
 		// "A writer MUST include exactly one `s` field."
 		let payment_secret_count = self.tagged_fields().filter(|&tf| match *tf {
 			TaggedField::PaymentSecret(_) => true,
@@ -1049,16 +1056,7 @@ impl Invoice {
 
 	/// Check that feature bits are set as required
 	fn check_feature_bits(&self) -> Result<(), SemanticError> {
-		// "A writer MUST include exactly one `s` field."
-		let payment_secret_count = self.tagged_fields().filter(|&tf| match *tf {
-			TaggedField::PaymentSecret(_) => true,
-			_ => false,
-		}).count();
-		if payment_secret_count < 1 {
-			return Err(SemanticError::NoPaymentSecret);
-		} else if payment_secret_count > 1 {
-			return Err(SemanticError::MultiplePaymentSecrets);
-		}
+		self.check_payment_secret()?;
 
 		// "A writer MUST set an s field if and only if the payment_secret feature is set."
 		// (this requirement has been since removed, and we now require the payment secret